1. Field of the Invention
The present invention relates to high power and/or low loss RF applications. More particularly, the present invention relates to high power and/or low loss RF applications where low loss, high performance coupling structures and a multi-layer PCB are required
2. Description of the Prior Art and Related Background Information
Directional coupling structures are well known in the art of RF design. These directional coupling structures are often used for sampling, combining or injecting signals. Directional couplers may be distinguished from other RF signal sampling, combining or injecting structures by their ability to sample (for example) power flowing in a particular direction while ignoring power flowing an opposite direction. A common metric for the directional coupler then is directivity—the ability of the coupling structure to distinguish between forward and reverse power flow. For a loose directional coupler to have high directivity, as is often required in RF systems, the even and odd mode wave propagation velocities must be equal. It is well known in the art that strip transmission lines utilizing a single type of dielectric are a good choice for implementing directional couplers with high directivity because the even and odd mode propagation velocities are inherently equal.
A strip transmission line comprises a conductor placed between two planar ground planes. If the conductor is suspended in the air between the ground planes, then the strip transmission line is often referred to as an “air dielectric strip transmission line.” An air dielectric strip transmission line is an excellent choice for low loss and good directional coupler performance. However, suspending the conductor between the planar ground planes is often difficult to realize using conventional printed circuit board (PCB) manufacturing techniques.
An air dielectric strip transmission line known in the prior art is disclosed in U.S. Pat. No. 5,712,607 to Dittmer et al. This structure provides low loss, but does not readily allow for a high performance (loose) directional coupler design. In particular, the odd mode propagation velocity will lag the even mode due to the dielectric material between edge coupled transmission lines. As disclosed in U.S. Pat. No. 4,394,630 to Kenyon et al., adding capacitance to the even mode may help to equalize the propagation velocities and hence improve the directivity of the coupler. While the method of compensation described by Kenyon et al. does improve directivity performance of a directional coupler, it is difficult to achieve the desired level of performance. Further, design time is extended due to the often difficult task of compensating coupling structures.
Accordingly a need exists for an improved air dielectric strip transmission line structure which overcomes the above noted problems.